Active noise reduction (ANR) devices have been commercially available for over 20 years. In general, these devices use electronics to generate a signal with the same amplitude but opposite phase of the noise. This is accomplished using a closed loop feedback control system having a sensing microphone to detect the noise with the associated signal passed through a compensating filter and electronics to drive a speaker that produces a pressure wave out of phase with the noise, resulting in a net reduction or attenuation of the noise perceived by a user.
Techniques for designing a feedback control system for active noise reduction are well understood by those skilled in the art. In general, the goal may be summarized as selecting components to provide system operating characteristics that satisfy control theory feedback loop stability criteria and provide a net attenuation or reduction of sound pressure at some or all of the frequencies of interest. This is accomplished by determining an appropriate open loop gain G, defined as the output/input ratio when the loop including the driver, sensing microphone, and electronics is driven and measured with the loop open, i.e. without feedback. G is a complex function, such that its magnitude and phase vary with frequency.
The corresponding attenuation provided by a system with open loop gain G can be expressed as 1/(1−G). In closed loop ANR circumaural designs having ear cups with a cushion that seals against the head around the circumference of the ear, this is typically limited to frequencies under 1 kHz. Because of a need for more attenuation of the lower frequencies, some boosting or amplification of the sound pressures is tolerated at higher frequencies where passive attenuation is more effective. In closed loop control systems, the amount of attenuation at lower frequencies is dependent on the acceptable phase margin around the upper transition frequency where the magnitude of the open loop gain (|G|) reaches unity. Phase margin is defined as the phase difference between the phase angle of the open loop gain (<G) and zero degrees when |G|=1. If the open loop gain has a magnitude close to unity and a phase of close to zero degrees, the denominator of 1/(1−G) will be much less than unity resulting in the function 1/(1−G) being much greater than unity at those frequencies and thus boosting of the pressure around those frequencies. Any compensation that causes a net decrease in amplitude with increasing frequency, has a resultant negative phase shift with more phase shift associated with steeper attenuation.
If 60 degrees or more phase margins can be maintained when the magnitude of the open loop gain (|G|) is close to unity, then no high frequency boosting will exist. Unfortunately, this generally produces inadequate loop gain at lower frequencies where passive attenuation is not as significant. Many designs accept some amount of high frequency boosting (making some frequencies louder when the ANR is on or active) to gain more attenuation at lower frequencies. In the design of such a system, transport or transit delay between the microphone input and driver output uses up valuable phase margin, and without changing the compensation, increases boosting around frequencies where the magnitude of G (|G|) is approximately unity. As a result, the sensing microphone has been placed in close proximity to the speaker (driver) to minimize delay as a result of the travel time of the sound to reach the microphone to provide acceptable phase margin and increase system bandwidth. In addition, the assumption of constant pressure within the front cavity of the ear cup of circumaural headphones at the frequencies the system attenuates also supports this approach as a good design methodology.
As such, use of well understood principles of feedback control system design and accepted operating assumptions have resulted in prior art systems that position the sensing microphone close to the speaker (also referred to as the driver) to maximize system bandwidth while providing acceptable phase margin for the system to remain stable and avoid unacceptable boosting of higher frequencies. The system parameters to provide acceptable phase margin are generally determined during product development based on average anatomical data and representative use scenarios. These parameters are generally fixed for the life of the product, or in some cases may be infrequently changed during firmware updates, but do not change during each use. While suitable for many applications, this design methodology does not account for variations among users with respect to ear anatomy as well as ambient environment.
Microprocessors and various dedicated purpose digital devices have afforded the opportunity for more complex digital processing of audio signals. However, processing speed remains an important consideration for real-time applications as any significant delay (on the order of 10 milliseconds) may produce an unacceptable lag, echo, distortion, or similar effect leading to an unnatural listening experience that may also affect speech patterns. Delay also imposes an inherent limitation to the bandwidth of broadband cancellation. The desire to avoid these effects may result in limiting the ANR performance over certain frequency bands.